<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <title>Title</title>
</head>
<body>
<input type="text" id="i1">
<input type="button" value="平方" onclick="f()">
<div>结果：<span></span></div>

<script>
  function f() {
    let i = document.querySelector("#i1");
    let s = document.querySelector("span");
    if(isNaN(i.value)){
      s.innerText="输入错误！重新输入";
      return;
    }
    /*s.innerText = i.value*i.value;*/
      //将parseFloat（）将字符转成整数或者小数
      s.innerText = parseFloat(i.value)+parseFloat(i.value);
  }

</script>
</body>
</html>